"I support giving arms to the government of Ukraine in order to help them protect themselves from the big Russian bear," the Florida Democrat told reporters Monday.

Nelson, a member of the Armed Services Committee, begins the trip later this week and will stop first in Istanbul, Turkey, for a policy conference, organized by the Aspen Institute Congressional Program, that will include a briefing on the Islamic State of Iraq and Syria. ISIS is the Islamic jihadist group that is being targeted by U.S. airstrikes in Iraq.

"We're going to be in this for months and months," Nelson said of the air campaign, echoing Obama. Asked if he is worried about the United States being drawn into a prolonged conflict, Nelson said ISIS must be contained.

"Of course, I'm worried. But I'm also worried about the very real possibility that if you don't stop ISIS, ISIS is going to take over that entire part of the world. … You're going to have a major new player in the world that is bent on the destruction of anyone who dares look different, talks different, acts different and thinks different. … The world doesn't need that kind of threat to its existence."

From Turkey, Nelson will go to Ukraine, where he will meet with President Petro Poroshenko, diplomats and other officials. He will also travel to Lithuania. "Those people are scared to death," of Russia, Nelson said.

Nelson's trip will last just under two weeks. Congress is on recess this month. Florida Sen. Marco Rubio, a Republican, has also argued for providing military assistance to Ukraine. Rubio, since June, had called on Obama to use airstrikes against ISIS.

Nelson said the United States has an interest in preventing Russia from advancing and breaking international law "by invading the sovereign territory of another country. Because if you start with one, Ukraine, then they'd go on to Georgia. They'd go on to the Baltic states."

In the past week, Ukrainian government forces have been closing on the few remaining pro-Russia strongholds in the east, including Donetsk, the largest rebel-held city.